The Lisp debugger

oo101 7 days ago [-]

There are a few ways in which the shared objects method you suggest does not match the full power of Lisp debugging.

First of all, when a C program crashes, it just crashes. There is no REPL. There is only a core dump. So any live-debugging you plan to do is after the fact. After you have seen a crash, you would now begin to prepare for the next crash by launching your process via GDB or restarting your process and attaching a GDB to it. Whether a similar crash would occur again or not or when it would occur again depends on the nature of the bug. Now contrast this with Lisp debugging when your program crashes, it stops there and offers you an REPL to interact with the program right then. There is no need to wait for the next crash.

Secondly, when you debug with GDB, you would be dealing with syntaxes: The syntax of C that we are so familiar with. The GDB syntax to investigate the problem that we may be less familiar with. When the Lisp debugger offers the REPL to you, you are working with Lisp again. Your compiler, debugger, program, etc. all are part of the same unified environment where you just execute Lisp code to debug your issue.

Finally, putting your code in shared objects and reloading them requires you to go through the complete write-build-test-debug cycle. And then what do you do if your shared object itself crashes? With Lisp you skip the write-build-test part when all you want to do is debug an error. You jump straight to the debug part of the cycle and begin investigating the runtime state. And it works the same in a uniform manner whether your main program crashes or a dependency crashes.

— A Road to Common Lisp

— Hacker News

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2022.05.05 Thursday ACHK